Molecular-Level Energy Storage

When the sun dips below the horizon for the night, most solar panels become interesting roofing tiles, instead of valuable generation resources. During the day, a single cloud can quickly send residential solar power generators back to a fossil fuel-based grid for their electricity. This intermittency in fuel resource availability, combined with the current lack of economic energy storage, impedes the ability of renewable energy technologies to compete with the existing fossil-fuel fleet. This provides a great opportunity area in research and development to find innovative solutions for storing energy. And, according to MIT researchers, one path to solving this competitiveness problem might be found at the molecular level – through their new understanding of fulvalene diruthenium.

Intermittency

The intermittent availability of fuel resources is not just a challenge for solar power. Other renewable power generation technologies suffer from the lack of control over fuel availability. In Texas – home to the most wind capacity in the nation – sudden drops in the West Texas wind have caused multiple electric grid emergencies. While Texas grid operators have become quite adept at managing these quick changes in wind power availability, including increased amounts of wind power in the generation mix has certainly added an additional level of complexity to the process.

When generation capacity is spread over large geographic areas – and connected to the nation’s electric grid - the intermittency of fuel resource availability can become a manageable problem. It is unlikely that clouds will cover a multi-state region for long periods of time, or that the wind will simultaneously stop blowing both on- and off-shore, for example. However, when communities wish to use local distributed generation, renewable fuel resource availability can become a more significant problem.

Energy Storage – Power and Heat

Problems associated with intermittent fuel resource availability are compounded by the current state of the energy storage market. Today, there are some limited technologies available for economic large-scale energy storage in the United States. For example, in Southern California they use pumped hydro (water) storage near the I-5 “grapevine” to store electricity during off-peak periods. In McIntosh, Alabama a 110 MW natural gas power plant is equipped with Compressed Air Energy Storage (CAES) to store off-peak power. And, in thermal storage, molten salts are being used in some limited projects. On a more exploratory level, investments are being made to determine the potential of flywheel technology (and many other possibilities) for large-scale energy storage.

But, for small-scale energy storage needs, the options are not only limited – but also expensive. Battery technology has yet to find the sweet spot for energy density – energy stored per unit of volume – at an economical cost. And, thermal energy storage is still largely in the R&D phase – the Ice Bear cooling system being one of the relatively few exceptions.

The Research Opportunity and MIT’s Discovery

Researchers throughout the world are working to figure out a way to inexpensively store large amounts of distributed energy. And, if (when) they are successful, these scientists could change the face of distributed generation by altering the economics of renewable energy technologies. One area of research focus – material science – specifically looks at the composition of different materials to figure out how they might be used to store energy. And, according to MIT researchers, the “solution” for energy storage might be found at the molecular level.

In partnership with colleagues at LLNL and UC Berkeley, a group of MIT researchers have discovered how a molecule called fulvalene diruthenium stores and releases heat on demand. According to their paper in Angewandte Chemie - International Edition,fulvalene diruthenium undergoes a structural transformation when it absorbs sunlight, putting it into a higher-energy state where it can remain stable indefinitely. By adding a small amount of heat or a catalyst, the molecule “snaps” back to its original shape, releasing the energy that it stored as additional heat.

"It turns out there’s an intermediate step that plays a major role… that was unexpected"

What is the importance of this unexpected intermediate step?

Essentially, this step results in the stability and reversibility that makes it possible to produce a “rechargeable heat battery” with this material. In this battery, we could store and release heat (thermal) energy. Dr. Grossman provides an explanation of the quantum mechanics related to this intermediate step in a short (1:47) YouTube video:

Fulvalene diruthenium has the ability (in theory) to store heat up to 200 degrees Celsius, which could be used directly to heat your home – kind of the opposite of the Ice Bear concept. While this “solution” would not address our electricity demand, it could offset some of our power demand for air and water heating.

But, before folks get too excited, I should note that fulvalene diruthenium is very expensive (and rare) and so is not itself a good candidate for cheap, abundant energy storage. But, understanding its behavior could lead to finding less rare materials that exhibit the same behavior. According to Dr. Grossman:

"[This] is the wrong material, but it shows it can be done…It’s my firm belief that as we understand what makes this material tick, we’ll find that there will be other materials [that work the same way]"

This might be the one we look back on and say “that was the moment that changed everything”… or maybe not… but either way, MIT’s determination of how a molecule called fulvalene diruthenium stores and releases heat on demand is pretty awesome.

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